Rotors of said type are known in the prior art in a variety of configurations and serve for converting forms of energy into one another in turbomachines. For example, the flow energy and/or enthalpy of a working fluid in a steam/gas turbine can be converted into rotational energy of a rotor (turbine rotor). Alternatively, a rotor driven in a rotating manner can be used to draw in an arbitrary gas, and to compress said gas for further use within an industrial process (compressor rotor).
Known rotors comprise a plurality of rotor segments which are each provided with a central opening and which are arranged axially adjacent to one another. Some of the rotor segments are in this case formed as so-called rotor disks, which each bear a ring of radially extending blades (rotor blades). Furthermore, such a rotor usually comprises a single central tie rod which extends through the openings of the rotor segments. Two bracing means which brace the rotor segments against one another are arranged at axially opposite ends of the tie rod.
During the operation of a turbomachine, the tie rod is caused to oscillate. Here, oscillation frequencies equal to or close to the natural frequency of the tie rod are to be avoided since such resonance oscillations of the tie rod can impair the function of the turbomachine or can lead to damage/destruction of the tie rod.
Turbine rotors are normally operated at a low rotational frequency, which substantially corresponds to the grid frequency of the respective power grid. The natural frequencies of the tie rods installed in turbine rotors are correspondingly generally well above said rotational frequency, and for this reason damaging resonance oscillations of the tie rod in turbine stages can scarcely occur.
The situation is different for compressor rotors since these are operated at rotational frequencies which are as high as possible. This is because the higher the rotational frequency, the greater the attainable compressor power. If the natural frequency of the tie rod of a compressor rotor must not be below the rotational frequency of the compressor rotor, this therefore constitutes a power-limiting factor for the compressor power.
Against this background, it is desirable to increase the natural frequency of the tie rod, in particular of a compressor rotor, which natural frequency is determined in principle by the dimensions and material properties of the tie rod and by the tensile force exerted on the tie rod with the aid of the bracing means. Here, said factors have different effects on the natural frequency of the tie rod.
The longer the free oscillation length of the tie rod, the lower the natural frequency of the tie rod. The plurality of rotor disks and further rotor segments may well result in a length for the compressor rotor, and thus also for the tie rod passing therethrough, which length is associated with a relatively low natural frequency of the tie rod, and this considerably limits the possible rotational frequencies of the compressor rotor.
By contrast, the larger the tensile force exerted on the tie rod via the bracing means, the higher the natural frequency of the tie rod. Consequently, the natural frequency of a tie rod can be increased by the rotor segments being braced against one another with greater intensity. However, the tensile force of the tie rod cannot be increased to an arbitrary extent since, by virtue of its material and dimensions, a maximum permissible tensile force must not be exceeded for the tie rod in order to avoid damaging or breaking the tie rod.
It is therefore not always possible in practice for a sufficiently high natural frequency of the tie rod to be structurally set in order to achieve a desired compressor power.
Said problem has hitherto been countered in that use has been made of different tie rod arrangements, which generally consist of multiple shorter, decentrally arranged tie rods. However, a disadvantage of this solution is that advantages associated with a single central tie rod, such as for example simple production and assembly, are no longer able to be realized.
Further rotors of the type mentioned in the introduction are described in the documents WO 2015/091436 A1, WO 2014/037521 A1 and JP 2006 138 255 A.